1. Field of the Invention
The present invention relates to a drive control device for causing an endless belt, drum member or similar endless movable member to perform adequate endless movement and a copier, printer, facsimile apparatus or similar image forming apparatus including the same.
2. Description of the Background Art
A drive control device for the above application is customary with a photoconductive drum, intermediate image transfer belt or similar endless movable member joining in an image forming process. When such an endless movable member is driven, it is necessary to accurately position an image on the surface of the movable member or a sheet or recording medium being conveyed by the movable member. It follows that the movement of the movable member for a unit period of time or the preselected point of the movable member at a preselected time must be controlled with high accuracy. In practice, however, the moving speed of the movable member is apt to vary due to various factors including a load exerted by a member contacting the movable member and cannot be fully controlled. It is therefore difficult to execute accurate control over the movement or the position of the movable member.
In light of the above, Japanese Patent No. 3,107,259, for example, discloses a control device configured to control the angular velocity of a drive source in accordance with the angular velocity of a photoconductive drum sensed by a rotary encoder, which is directly connected to the shaft of the drum. Because the photoconductive drum is affixed to the shaft, the moving speed of the surface of the drum and the angular velocity of the shaft are not shifted from each other. Therefore, the control device can execute accurate drive control with a member affixed to a shaft like the photoconductive drum.
However, the control device taught in the above document does not execute drive control on the basis of the movement or the position of the drum, which is the subject of control. Accurate drive control is not available with the control device when a photoconductive belt or an intermediate image transfer belt or similar endless belt member is not directly connected to a drive shaft driven by a drive source.
On the other hand, Japanese Patent Laid-Open Publication Nos. 9-114348 and 6-263281, for example, each disclose a drive control device of the type forming marks on the outer or the inner surface of an endless belt member and feeding back the output of a mark sensor responsive to drive control. The drive control devices taught in these documents directly observe the behavior of the belt member itself and can therefore execute more accurate drive control than the control device of Japanese Patent No. 3,107,259.
More specifically, the drive control device taught in Laid-Open Publication No. 9-114348 includes a mark sensor responsive to a plurality of marks formed on a sheet conveying belt at preselected intervals in the direction of movement of the belt. The drive control device controls the drive of the belt in accordance with data produced by sampling the output of the mark sensor. More specifically, the drive control device calculates the distance of movement of the belt and a mean speed in a preselected period and controls the drive of the belt in accordance with the calculated distance and mean speed.
The drive control stated above is effective so long as signals are output at preselected intervals like the outputs of a rotary encoder. However, it is extremely difficult to form marks on the belt member at preselected intervals although the document does not show or describe a mark forming method specifically. For example, when the belt member is produced by a mold formed with projections and recesses for forming the marks, the belt member is generally pulled out of the mold and then subject to annealing. If the belt material is not uniformly heated during annealing, then the contraction ratio of the entire belt becomes irregular and prevents the distance between nearby marks from being uniform. Moreover, strain produced in the belt member after molding makes the contraction ratio and therefore the distance between nearby marks irregular.
To form marks on an endless belt member, the marks may be printed, adhered or otherwise put on the belt member. When the marks are so put on the belt member after molding, the non-uniform contraction distribution of the belt member does not effect the distance between nearby belts. However, as for the production of endless belt members, the tolerance of circumference length is generally selected to fall between 0.2% and 0.3% or so. Therefore, if the circumference of a belt member is 500 mm long, then the tolerance amounts to 1 mm or above. Consequently, some of belt members produced differ in circumferential length from the other belt members by 1 mm or more. Such a difference in circumferential length makes it extremely difficult to connect a seam portion between the beginning and the end of continuous marks such that the seam portion has the same interval as the continuous mark portion.
In the above circumstances, the continuous marks include a discontinuous portion in which the distance between nearby marks differs from the distance between the other marks. The discontinuous portion directly translates into a mark sensing error or unstable drive control. When a PLL (Phase Locked Loop) circuit is used to cause an endless belt member to move at constant speed, a reference signal and a comparison signal derived from the marks are compared in phase in the PLL circuit. At this instant, if a mark sensing error occurs or if mark sensing timing is noticeably shifted, then the phase of the reference signal and that of the comparison signal are noticeably shifted from each other, resulting in unstable control. This problem arises even when the endless drive member to be controlled is implemented as a drum member.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 2002-108169, 2002-136164 and 2002-238274.